Graphic arts applications frequently require the accurate reproduction of a high resolution color image (commonly referred to as an "artwork"), such as a color photograph, a color drawing, a color layout and the like. A typical application might involve printing a high resolution color image or a series of such images on a page of a periodical, such as a magazine, or a corporate annual report.
Images are oftentimes generated either photographically, on suitable film, or electronically, on video tape or other suitable electronic media. When generated, images share a basic characteristic: they are recorded on a continuous tone (hereinafter referred to as "contone") basis. As such, the color existing at any point in the image is represented by a continuous amplitude value.
Color reproduction equipment takes advantage of the principle that the vast majority of colors can be separated into a specific linear combination of four primary subtractive colors (cyan, yellow, magenta and black--C, Y, M and B) in which the amount of each primary color is set to a predetermined amount. In the case of printed reproductions of an image, use of primary color printing obviates the need to use a differently colored ink for each different color in the image. As such, each image is converted into a succession of three or four color separations, in which each separation is essentially a negative (or positive) transparency with an altered tone reproducing characteristic that carries the color information for only one of the primary colors.
Unfortunately, modern printing presses do not possess the capability of applying differential amounts of ink to any location in an image being printed. Rather, these presses are only designed to either apply or not apply a single amount of ink to any given location on a page. Therefore, a printing press is unable to directly print a contone separation. To successfully circumvent this problem, halftone separations are used instead. An image formed from any single color halftone separation encodes the density information inherent in a color image from amplitude modulated form into a spatial (area) modulated form, in terms of dot size, which is subsequently integrated by the human eye into a desired color. By smoothly changing dot sizes (areas), smooth corresponding tonal variations will be generated in the reproduced image. Given this, the art has taught for some time that a full color image can be formed by properly overlaying single color halftone reproductions for all of the primary subtractive colors, where each reproduction is formed from a halftone dot separation that contains dots of appropriate sizes and in one of these primary colors. Clearly, as size of the dots decreases, an increasing amount of detail can be encoded in a dot pattern and hence in the reproduced image. For that reason, in graphic arts applications, a halftone separation utilizes very small dots to yield a relatively high dot pitch (resolution).
With this in mind, one might first think that printing a color image for graphic arts use should be a fairly simple process. Specifically, a color image could first be converted into corresponding continuous tone separations. Each of these contone separations could then be converted into a corresponding halftone separation. A printing plate could then be manufactured from each halftone separation and subsequently mounted to a printing press. Thereafter, paper or other similar media could be run through the press in such a fashion so as to produce properly registered superimposed halftone images for all the subtractive primary colors thereby generating a full color reproduction of the original image.
Unfortunately, in practice, accurately printing a color image is oftentimes a very tedious, problematic and time consuming manual process that requires a substantial level of skill. First, the conventional manual photographic process of converting a contone separation into a halftone separation, this process commonly being referred to as "screening", is a time consuming manual process in and of itself. Second, various phenomena, each of which disadvantageously degrades an image, often occur in a reproduced halftone color image. Moreover, the complete extent to which each of these phenomena is present in the reproduced image is often known only at a rather late point in the printing process thereby necessitating the use of tedious and time consuming iterative trial and error experimentation to adequately eliminate these phenomena.
Specifically, to verify the accuracy of the color printing process and to enable appropriate adjustments to be made at various stages in the printing process in order to correct image defects and improve reproduction accuracy, a test image, frequently referred to as a "proof" is generated from the halftone separations once they are made. After a proof is generated, it is presented as being representative of the reproduced image which will be produced by a printing press in order to determine the accuracy of the printed image. Oftentimes, the proof contains unexpected and unsightly Moire patterns that arose from the interaction of Moire in the image itself with that introduced by use of angled halftone screens that are used to photographically generate the halftone separations. Frequently, these Moire patterns can be rendered invisible by rotation of one or more of the screens to a different screen angle. Unfortunately, the exact change in the screen angle is frequently very hard to discern from the resulting Moire pattern itself and instead must be determined through trial and error experimentation. Unexpected artifacts can also exist in the proof thereby necessitating that various changes must be made to one or more of the separations. As such, this requires that a one or more new halftone separations must be generated or at least changed, a new proof must be produced and then analyzed, with this "proofing" process being iteratively repeated until the objectionable Moire and all objectionable artifacts are eliminated from the proof. Now, once an acceptable proof is made thereby indicating that a printed image based on the separations will likely present a desired depiction of the original artwork, a separate printing plate is then made for each halftone separation. At this point, a full color test print, commonly referred to as a "press sheet" is produced from these plates onto a sheet of actual paper stock that is to be used to carry the reproduced image, with this operation frequently being referred to as a "press run". The press sheet is then examined to discern all imperfections that exist in the image reproduced therein. Owing to unexpected dot gain, existence of any artifacts in the press sheet and tonal variations occurring in the press run between the press sheet and the original artwork, further adjustments in the coloration or screen angle of the separations may need to be made with the entire process, i.e. both the proofing and the press run processes, being repeated until an acceptable press sheet is produced. With experience gained over several years, a skilled color technician can reduce the number of times that this entire process needs to be repeated in order to produce a set of color halftone separations that yields an acceptable press sheet.
As one can now readily appreciate, the iterative manual process of producing an acceptable set of halftone separations, due to the inherent variability of the process, can be very tedious and inordinately time consuming. Unfortunately, in the graphic arts industry, publication deadlines are often extremely tight and afford very little, if any, leeway. Consequently, the available time in a graphic arts production environment allotted to a color technician to generate a set of halftone separations to meet a particular publication deadline, for example, is often insufficient to allow the technician adequate time, due to the trial and error nature of iterative process, to generate that set of separations which produces a very high quality halftone color image. As such, the technician is often constrained by time pressures to produce a set of separations that produces a visually acceptable, though not necessarily a very high quality, image.
In addition, the manual process can be disadvantageously quite expensive. Inasmuch as the manual process, even for a skilled color technician, involves a certain amount of trial and error experimentation, a number of separate proofs is often made with changed or new separations being generated as a result. Each new separation requires another piece of film. Film and associated developing chemicals are expensive. In addition, if an unacceptable press sheet is produced, then additional separations may need to be made along with new printing plates, which further lengthens the process and increases its expense.
In an effort to reduce the time required and expense associated with conventional manual photographic based color reproduction processes, the art has turned away from use of these manual processes in high volume graphic art applications to the use of other technologies, such as electro-photographic techniques. In this regard, U.S. Pat. No. 4,708,459 (issued to C. Cowan et al on Nov. 24, 1987 and assigned to the present assignee hereof) discloses an electro-photographic color proofing system. While this system does produce an excellent quality proof, the throughput of proof images provided by this system, while far in excess of that associated with manual proofing techniques, has nevertheless proven to be somewhat inadequate for use in those segments of the graphic arts industry that routinely experience tight publication deadlines.
Accordingly, in an effort to provide an even greater throughput, so-called direct digital color proofing (DDCP) systems are being envisioned for directly generating a halftone color proof image from digitized contone separations. These systems would manipulate the separations in digital form to electronically generate appropriate halftone separations, including, inter alia, electronic screening and dot gain compensation, and then directly write the proof image using an appropriate marking engine (henceforth referred to as a "proofing engine").
Unfortunately, DDCP systems, as typically envisioned by those skilled in the art, would probably suffer from two drawbacks that could limit both their throughput and ease of use.
First, these DDCP systems would probably produce proof images based upon "spooling" a sequence of electronic proof requests. A proof request in such a system would be expected to contain both appropriate instructions and parameters, for the proofing engine, to generate each separate proof as well as the accompanying halftone image separation data itself. Unfortunately, since the data for each halftone image separation represents a bit-map of the separation at, what is often, a very fine resolution, a substantial amount of memory space, typically on the order of 20 Mbytes, would be required to store each halftone separation. An additional 20 Mbytes may also be required for linework that is to be printed as part of the image. Hence, for a single four color image, the resulting data for all the separations and the linework in a proof request would require upwards of typically 100 Mbytes of storage space. In a spooled environment, each of these proof requests would be entered into a print queue stored on a hard disk from which these requests would be read and serviced by the proofing engine typically on a first-in, first-out (FIFO) basis. As a consequence of allocating 100 Mbytes for each request, a very substantial amount of hard disk space would be required to accommodate this queue. Hence, owing to finite limits on available hard disk space, the queue could only accommodate a relatively small number of proof requests at any one time. Consequently, to operate the DDCP system at its maximum throughput, an operator would need to continually and repetitively enter new proof requests at relatively short intervals as the queue emptied. This, in turn, places a burden on the operator. Furthermore, if these requests were to be transferred from one point to another within the DDCP system, then an appreciable amount of time would be needed just to transfer the large amount of accompanying image data which, in turn, would suppress the throughput of the DDCP system.
Second, the DDCP system itself would need to be properly configured with appropriate parameter values to generate each individual proof, such as for example with proper screen angles and screen rulings for each separation and the proof image resolution and dimensions. As such, one would think that a human operator would be delegated the task of entering the parameters particular to each proof image into the DDCP system to form each proof request. Unfortunately, this task would place a significant burden on the DDCP operator. Furthermore, the reliance on such an operator to manually enter each proof request into the DDCP system may, through inadvertent human error, cause a significant source of proofing errors which, in turn, would force the operator to expend time and system resources to locate each of these errors and re-proof each affected image to generate an acceptable proof image. This, in turn, would also suppress the throughput of acceptable proof images of the DDCP system.
Therefore, a need exists in the art for a technique for inclusion in a direct digital color proofing system that can significantly increase the throughout of such a system while simultaneously reducing the amount of interaction needed between the operator and the system to correctly generate a sequence of proof images.